Internal gear pumps are used, for example, as oil pumps for lubrication of a car engine and for an automatic transmission (AT). In some pump rotors adopted in the internal gear pumps, inner and outer rotors, whose numbers of teeth are different by one, are combined. Further, in some rotors of this type, the tooth profile of the rotor is formed by a trochoidal curve, or the tooth profile of the rotor is formed by a cycloidal curve.
As shown in FIG. 15, a tooth profile using a trochoidal curve is formed using a base circle E and a rolling circle F that does not slip, but rolls on the base circle E. More specifically, a trochoidal curve TC is drawn by a locus of one point on a radius at a distance e (=amount of eccentricity between the centers of an inner rotor and an outer rotor) from the center of the rolling circle F, and a tooth profile of an inner rotor 2 is formed by an envelope of a group of arcs of a locus circle G that moves on the trochoidal curve TC, has the center on the trochoidal curve, and has a fixed diameter (see the following Patent Document 1).
As for a tooth profile defined by a cycloidal curve, a tooth profile of an inner rotor is formed by a base circle, a locus of one point on the circumference of an externally rolling circle that does not slip, but rolls on the base circle while being circumscribed about the base circle, and a locus of one point on the circumference of an internally rolling circle that does not slip, but rolls on the base circle while being inscribed in the base circle.